1. Field of the Invention
The present invention relates to broadband amplifiers for use in, e.g., television tuners.
2. Description of the Related Art
FIG. 11 shows a broadband amplifier of the related art (see Japanese Unexamined Patent Application Publication No. 08-274548, FIG. 1). An input terminal RFin is coupled to a ground via a coupling capacitor Cin and an input resistor r1. A node a between the coupling capacitor Cin and the input resistor r1 is connected to a gate of a field-effect transistor (FET) Q1. The source of the FET Q1 is coupled to the ground via a resistor r2 or a capacitor C1. The drain of the FET Q1 is connected to an output terminal RFout via an output capacitor Cout. The drain of the FET Q1 is also connected to a drain power supply VDD via a bias choke coil L1.
A feedback resistor section 11 is connected between the node a and a node b between the drain of the FET Q1 and the bias choke coil L1. The feedback resistor section 11 includes a DC (direct current) cut capacitor C2, a PIN diode D1, a variable resistor r3, and a DC cut capacitor C3, which are connected in series from the node b. A node between the PIN diode D1 and the DC cut capacitor C2 is connected to a gain control power supply VC via a bias choke coil L2. A node between the PIN diode D1 and the variable resistor r3 is coupled to the ground via a choke coil L3.
In this structure, the resistance of the PIN diode D1 is changed by controlling the voltage of the gain control power supply VC, thereby changing the amount of feedback and therefore changing the gain. Specifically, when the voltage of the gain control power supply VC increases, the resistance of the PIN diode D1 decreases and the amount of feedback becomes large, resulting in a reduction in the gain. When the voltage of the gain control power supply VC decreases, the resistance of the PIN diode D1 increases and the amount of feedback becomes small, resulting in an increase in the gain.
The broadband amplifier shown in FIG. 11 changes only the amount of feedback to change the gain, and does not control the noise figure (NF) characteristic and the distortion characteristic depending upon the application of use.